Immunoassay is a measuring method which utilizes the characteristics of specifically binding antibody to the target substance as antigen. As a representative immunoassay, enzyme immunoassay (EIA) is generally known. The measurement sensitivity in EIA is high to be in a few ng/mL level, and EIA is widely used as a method of testing carcinoma-associated proteins, viruses and the like, in a study, a clinical examination and the like. Further, recently, more highly sensitive assays receive much attention, because of an item requiring a measurement sensitivity of 1 ng/mL or less, a small amount of test samples to use, and the like, so that chemiluminescent enzyme immunoassay (CLEIA) method has started to become popular (for example, see Patent Documents 1 and 2).
In conventional enzyme immunoassay and chemiluminescent enzyme immunoassay, a method is adopted in which a specific antibody selectively binding to the target substance such as protein is immobilized to a vessel such as a test tube or a 96-well plastic microplate; plastic beads; or magnetostrictive material such as ferrite, and the target substance is selectively taken out from a liquid layer. Such a method in which a reaction is carried out in a test tube, microplate or the like, and the color tone or fluorescent substance or luminescent substance appeared in a liquid layer is measured, is referred to as liquid-phase type assay method, and has problems that the assay procedures such as injection of reagents and drainage are troublesome and that the assay equipment is large.
On the other hand, non-liquid phase type immunoassay methods are also developed, and as a representative, there is an immunochromatography method. Immunochromatography is the method of measuring the target substance by intensively capturing an immune complex of the target substance and labeled antibody (labeled with a visualizing substance such as gold colloid particle and color latex) in a section linearly arranged with antibodies on a test device, and visually or optically reading the captured immune complex (appeared as a colored line) (for example, see Patent Document 3). In the immunochromatography method, approximate concentration of the target substance is obtained only by observing the appeared line after applying a sample such as urine and blood into a sample applying section, and thus the operation is easy. The sample volume required for the immunochromatography method is about 100 μL, but the sensitivity for the measurement is about 1 ng/mL.    Patent Document 1: Japanese Patent Laid-Open Publication No. H03-53897    Patent Document 2: Japanese Patent Laid-Open Publication No. 2001-249079    Patent Document 3: Japanese Patent No. 3481894